For example, in plants for manufacturing automobile bodies, a spot weld inspection method which can be easily carried out has been recently desired for inspecting spot weld zones with high efficiency on site.
Since an automobile body is assembled by spot welding at thousands of points, quality of spot welding directly influences the strength and durability of the body, and thus it is very important to inspect whether or not spot welding is properly performed. As a method of inspecting spot weld zones, chisel inspection has been carried out for deciding the quality by confirming whether or not a spot weld zone is separated by a cold chisel inserted between spot-welded metal sheets. However, in the chisel inspection, spot weld zones may be broken, and thus it is difficult to precisely determine the quality of spot welding by the chisel inspection. Also, members each having a spot weld zone broken by the chisel inspection cannot be used for products, and there is thus the problem of increasing the cost.
Therefore, various apparatuses and methods have been recently proposed for ultrasonic nondestructive inspection of the quality of spot welding.
For example, Japanese Unexamined Patent Application Publication Nos. 2000-146928, 2002-131297, 11-2627, and 6-265529 disclose methods and apparatuses in each of which an ultrasonic wave is vertically incident onto a sheet surface to detect a reflected wave, for evaluating the quality of a spot weld zone formed by welding two stacked sheets. Japanese Unexamined Patent Application Publication No. 62-52456 discloses an ultrasonic flaw detector based on local immersion method in which a pair of immersion probes disposed on and below a test piece to be opposed to each other with the test object disposed therebetween, and the test object is horizontally moved so that a spot weld zone of the test object is scanned by an ultrasonic beam transmitted from the transmitting probe and the presence of a flaw in the spot weld zone is determined on the basis of a signal received by the receiving probe.
In the above-mentioned prior art, an ultrasonic wave is sent and received vertically to the plate-shaped test object. Therefore, in an inclined surface 102 formed around each dimple 102b formed in a spot weld zone 102 of a test object illustrated in FIG. 10, an ultrasonic beam cannot be efficiently incident onto the test object. There is thus the problem of difficulty in precisely detecting the size of a nugget 102a formed in the spot weld zone 102.
In other words, as shown in FIG. 10, when an upper sheet 101a and a lower sheet 101b are stacked and welded together by spot welding, a weld solidified structure 102a referred to as a “nugget” is formed in the spot weld zone 102 at a joint between the upper and lower sheets 101a and 101b. In spot welding, the upper and lower sheets 101a and 101b are strongly pressed by electrode tips not shown in the drawing to form the dimples 102b corresponding to the shapes of the electrode tips in the surfaces of the upper and lower sheets 101a and 101b. 
Furthermore, the conical inclined surfaces 102c are formed between the bottoms of the dimples 102b and the surfaces of the upper and lower sheets 101a and 101b. In normal welding, the diameter of the nugget 102a is slightly larger than or substantially the same as the diameter of the electrode tips used for welding. Since the electrode tips have a chamfered cylindrical tip shape, the inner diameter of the dimples 102b is slightly smaller than the diameter of the cylindrical portions of the electrode tips. Therefore, the inner diameter of the dimples 102b is generally slightly smaller than the diameter of the nugget 102a. In abnormal welding, the diameter of the nugget is smaller than that in normal welding, thereby causing abnormality such as lack of strength. In FIG. 10, symbol S represents the toe of the nugget 102a. The toe refers to a point of intersection between the front or back surface of the upper sheet 101a or the lower sheet 101b and the boundary of a weld metal.
As described above, in the spot weld zone 102, the conical inclined surfaces 102c are formed between the bottoms of the respective dimples 102b and the surfaces of the upper and lower sheets 101a and 101b. Therefore, an ultrasonic wave is reflected by the inclined surfaces 102c and little transmitted into the test object, thereby obtaining substantially no signal from a portion to be inspected by use of an ultrasonic inspection apparatus of the prior art in which an ultrasonic beam is sent and received vertically to the surfaces of the upper and lower sheets 101a and 101b of a test object. As described above, the size of the nugget 102a is slightly larger than or substantially the same as the diameter of the electrode tips, and thus the toe S of the nugget 102a substantially overlaps the inclined surfaces 102c formed in the test object. Therefore, as an ultrasonic wave is reflected by one of the inclined surfaces 102c, a correct signal cannot be obtained from a portion near the nugget toe S, thereby causing difficulty in precisely determining the nugget diameter and deciding whether or not a defect such as a crack or the like is present.
In Japanese Unexamined Patent Application Publication No. 2004-163210, the inventor of the present invention has already proposed an ultrasonic method of evaluating a spot weld zone formed by welding a plurality of stacked metal sheets, in which a Lamb wave is excited in the metal sheets outside the spot weld zone toward a weld metal, and the Lamb wave is transmitted through the weld metal and then received after the transmission to evaluate soundness of the spot weld zone. In this method, evaluation of the spot weld zone was succeeded without being influenced by the inclined surfaces formed around the dimples formed in the spot weld zone. However, in Japanese Unexamined Patent Application Publication No. 2004-163210, it was found that a deviation of the positional relation between the two Lamb wave probes and the spot weld zone from a predetermined positional relation, which may happen in disposing two Lamb wave probes opposite to each other with the spot weld zone disposed therebetween, causes a deviation of the Lamb wave propagation path from the center of the spot weld zone, thereby failing to correctly evaluate the soundness of the spot weld zone. This problem significantly occurs when the relative positional relation between the Lamb wave probes and the spot weld zone cannot be sufficiently controlled because the measurement time is limited to a short time.
The Lamb wave is also referred to as a “plate wave” and is produced by oblique incidence of an ultrasonic wave onto a thin plate (a metal sheet or a non-metal sheet) at a specified angle of incidence. Obliquely traveling longitudinal and transverse waves generated in a thin plate from oblique incidence refraction propagate and interfere with each other while repeating reflection followed by mode conversion at the front and back surfaces of the thin plate, thereby producing a traveling wave displaced symmetrically or asymmetrically with respect to the center of the thin plate in the thickness direction. This traveling wave is the Lamb wave (refer to Joseph L. Rose, Ultrasonic waves in solid media, pp. 101-126, Cambridge Univ. Press, Cambridge, 1999). A Lamb wave probe is an ultrasonic probe for making an ultrasonic wave incident on a thin plate at a specified angle of incidence in order to excite a Lamb wave in the thin plate. The Lamb wave probe can also be used for receiving Lamb waves.
It was further found that in evaluating the soundness of a spot weld zone using the method disclosed in Japanese Unexamined Patent Application Publication No. 2004-163210, the soundness of a spot weld zone cannot be precisely evaluated in some cases because the amplitude of a transmitted Lamb wave may be changed depending on a change in the coupling condition between two Lamb wave probes and a metal sheet unless a method of detecting the amplitudes A1 and A2 of transmitted Lamb waves at two frequencies f1 and f2 (f1<f2) shown in claim 3 of Japanese Unexamined Patent Application Publication No. 2004-163210 and determining the ratio (A2/A1) is used. This problem significantly occurs when the coupling condition between two Lamb wave probes and a metal sheet cannot be sufficiently stabilized because the measurement time is limited to a short time. In order to use Lamb waves at two frequencies, it is necessary to arrange two piezoelectric elements in a line in a Lamb wave probe because of the need for the Lamb waves to have the same passage path. Therefore, a Lamb wave probe is inevitably increased in size, thereby causing the problem of difficulty in evaluating a spot weld zone in a narrow portion.
It could therefore be advantageous to evaluate the soundness (the presence of a nugget, the nugget diameter, and weld cracking) of a spot weld zone with high reliability within a short measurement time (for example, 5 seconds or less per spot) without the influence of a deviation between the positions of an ultrasonic probe and the spot weld zone and a coupling condition between an ultrasonic probe and a metal sheet.